Field of the Invention
Embodiments of the present invention relate generally to a multi charged particle beam exposing method and a multi charged particle beam exposing apparatus, and more specifically, for example, to a beam irradiation method in multi-beam writing.
Description of Related Art
The lithography technique that advances miniaturization of semiconductor devices is extremely important as a unique process whereby patterns are formed in semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits becomes progressively narrower year by year. The electron beam writing technique, which intrinsically has excellent resolution, is used for writing or “drawing” patterns on a wafer and the like with electron beams.
As a known example of employing the electron beam writing technique, there is a writing apparatus using multi-beams. Since it is possible for multi-beam writing to irradiate multiple beams at a time, the writing throughput can be greatly increased in comparison with single beam writing. For example, a writing apparatus employing the multi-beam technique forms multi-beams by letting portions of an electron beam emitted from an electron gun pass through a corresponding hole of a plurality of holes in a mask, performs blanking control for each beam, reduces each unblocked beam by an optical system, and deflects it by a deflector so as to irradiate a desired position on a target object or “sample”.
In multi-beam writing, the dose of each beam is individually controlled based on an irradiation time. A control circuit for performing an individual control is included in a blanking aperture array apparatus. The irradiation time of each beam is individually generated as N-bit irradiation time data. A beam ON time of each beam is controlled by individually counting a clock period by using an N-bit control circuit arranged for each beam.
FIG. 12 shows an example of relation among the number of counter bits, multiplicity, and a data transmission amount. The comparative example 1 in FIG. 12 shows the case where an N-bit control circuit is arranged for each beam of multi-beams, and an exposure is performed with the multiplicity 1 of exposing times by using N-bit irradiation time data, for example. However, as the number of beams of multi-beams increases, the pitch between beams becomes narrower. Moreover, if the number of bits used in the control circuit increases, the size of the circuit itself also increases in proportion to the number of bits. Therefore, if the number of bits (N bits) used as irradiation time data becomes large, it will be difficult to arrange the N-bit control circuit for each beam. Actually, with the increase in the number of beams of multi-beams, it has become difficult to define a required irradiation time by using n bits being the number of bits with which the control circuit can be arranged. Therefore, it may be possible to reduce an irradiation time and increase the number of times of irradiation (multiplicity m) to compensate the reduction so that the irradiation time can be defined based on n (n<N) bits being the number of bits with which the control circuit can be arranged. When a required irradiation time can be defined by 1023 gray levels (N=10 bits) at maximum, for example, if the number of bits, n, of the control circuit to control an irradiation time is defined using n=N (10 bits), for example, as shown in the comparative example 1 of FIG. 12, it becomes possible to define the irradiation time up to 1023 gray levels at maximum. Therefore, the required irradiation time can be exposed with multiplicity m=1. The data transmission amount in that case is N×m=10 bits. However, as described above, with the increase in the number of beams, it is becoming difficult to arrange the N-bit control circuit. On the other hand, as shown in the comparative example 2 of FIG. 12, if the number of bits, n, of the control circuit to control an irradiation time is defined by, for example, 5 bits, the irradiation time can be defined up to 31 gray levels at maximum. However, for acquiring an irradiation time nearly equivalent to 10 bits, multiple exposures of 32 times (multiplicity m=32) are needed. Even in such a case, the irradiation time can be merely defined up to 960 gray levels at maximum as the total of multiple exposures of 32 times. However, since 5 bits are needed for irradiation time data for one irradiation, the data transmission amount is n×m=5 bits×32 times=160 bits. Thus, when multiple exposures are performed, there is a problem in that the data transmission amount increases.
With respect to the above, there is proposed a method of controlling an irradiation time not by individually counting the irradiation time for each beam, but by dividing a shot of a maximum irradiation time irradiating the same position into a plurality of irradiation steps which are common to all the beams of multi-beams and each of which has a different irradiation time, selecting for each beam only a required irradiation step from a plurality of irradiation steps, and making a beam ON only at the selected irradiation step in order to control, for each beam, the irradiation time based on a total of combination of irradiation steps of beam ON (for example, refer to Japanese Patent Application Laid-open No. 2015-002189). This method employs a control system of controlling ON or OFF based on a timing signal for each irradiation step, without mounting a counter circuit in the blanking aperture array apparatus.
The problem described above with respect to the control method of the irradiation time of each beam of multi-beams is not limited to a writing apparatus, and may occur in any apparatus which performs exposure by irradiating a target object with multi-beams.